This error occurs when one of the print cores is reading a temperature that is too low. The display will show with which print core a problem has been detected.
One of the reasons for this could be the environmental temperature, so always make sure that you're not printing in an environment that is too cold. The recommended ambient operating temperature is at least 15 ºC (59 ºF).
If you're experiencing this error message while the environmental conditions are correct, there probably is a problem with the print core PCB located at the back of the print core, or the PT100 temperature sensor in the print core heater block. In this case the print core cannot be fixed and must be replaced. Please contact your local service provider for assistance.